1. Field of the Invention
This invention relates to film scanners and more particularly to the optical configuration of a film scanner employing a line sensing array for scanning in one direction and a rotating multifacet mirror such as a reflecting polygon for displacing the line scan in a direction perpendicular to the line scan direction to effect a raster scan of the film.
2. Discussion Related to the Problem
Apparatus for scanning motion picture film to produce a television signal is well known. In such film scanning apparatus, a film frame is scanned in a direction generally perpendicular to the length of the film at a standard television line rate. The line scan is displaced in a direction generally parallel with the length of the film at a standard television field rate. In the prior art, the line scan has been accomplished by flying spot scanners employing Nipkow discs, movable reflecting elements or cathode ray tubes. The use of a solid-state line sensing array as an alternative to a flying spot scanner to provide the line scanning function in a film scanner has been proposed. See D. T. Wright, "Solid-State Sensors; The Use of a Single Dimension 512-Element Array for Film Scanning," BBC Research Department Report No. 1973/32. The potential advantages of a line sensing array over a flying spot scanner are numerous, including: reduced size, weight, maintenance and cost.
These advantages make the solid-state line sensing array particularly attractive for use in a film scanner for displaying amateur movie film on a home television set. A technique for converting an 18 frame per second motion picture film to a 60 field per second television signal is disclosed in copending U.S. patent application No. 68,032 entitled FILM SCANNING METHOD AND APPARATUS EMPLOYING OPTICAL SPLICE TECHNIQUE FOR FRAME-RATE CONVERSION, by D. G. Howe filed on the same day as this application. The scanning apparatus disclosed by Howe includes a solid-state line sensing array for scanning a given line of a projected image of the film, in a direction generally perpendicular to the length of the film, and a rotating multifacet mirror comprising a reflecting polygon for displacing the image relative to the line sensing array, in a direction generally parallel to the length of the film to effect a raster scan of the film frames. The design of the optical configuration of the rotating polygon scanner presented the following problems.
If the scanning objective lens is positioned in the optical path between the film and the rotating polygon so that the optical axis of the lens is deflected by the polygon facets to sweep across the line sensor, most of the time, the image falling on the line sensor is an "off-axis" image. This requires a very high quality lens to achieve acceptable resolution. Furthermore, if the portion of the image impinging upon the line sensing array is to remain in focus, an anamorphic (cylindrical field) lens must be employed. As is well known, a high quality, anamorphic lens of this type would be very expensive to manufacture. The cost of such a lens would constitute a major portion of the total cost of the scanner.
On the other hand, if the scanning objective lens is positioned in the optical path between the rotating polygon and the line sensing array so that the optical axis of the lens is deflected by the polygon to sweep the film gate, the image falling on the line sensor is always on-axis in the direction of the length of the film and a relatively simple, and consequently less expensive, lens may be used. Unfortunately, this scanning arrangement introduces other problems. In general, with the few exceptions noted below, the film surface, required to keep the portion of the image impinging on the line sensing array in focus as the image is displaced by the polygon, will not be a flat plane, or any other simply realizable configuration for a film plane. In fact, the shape of the surface, as viewed through a cross-section taken parallel with the length of the film, will generally resemble a recurved bow. Other types of optical distortion and vignetting are encountered as a function of the changing angle that the optical axis makes with a facet of the polygon as the polygon rotates. The angularly dependent distortion and vignetting generally increase as the angle between the optical axis and a polygon facet varies from 90.degree..
The problem therefore is to provide an optical scanning system for a rotating polygon film scanner that employs inexpensive optics (like the latter mentioned scanning arrangement) while providing an easily defined film surface and that also minimizes the angularly dependent optical distortion and vignetting caused by the rotating polygon scanner.
It also is desirable that such a system work with a relatively fast (i.e. low f/no.) scan lens. A relatively fast scan lens allows the line sensor to be irradiated with enough light so that the sensor will produce signals having a good signal-to-noise ratio when the sensor is operated at rates corresponding with TV line scan rates.